A conventional pump, as shown in FIG. 5 of the attached drawings, comprises a front casing 100 attached to a rear casing 200 to define an interior space 101 for accommodating an impeller 300. The impeller 300 is supported by a shaft and driven by a motor to rotate in the interior space 101. A trust 500 is fixed in an inside surface of the front casing 100. A wearing ring 400 is attached to the impeller 300 for contactingly engaging the thrust ring 500. Since there is always essential clearance between the shaft and bushing, the rotation of the impeller 300 is unavoidably an eccentric motion. This causes a non-uniform engagement between the thrust ring 500 and the wearing ring 400, leading to breakage of lubrication film therebetween and eventually shorting the service life time of wearing ring 400.
Thus, it is desired to provide a pump structure for overcoming the problems.